Conventionally, when a mobile radio apparatus such as a mobile telephone and a PHS is placed in a so-called laid state with its back surface contacting a metal top panel of a steel desk and so on, its antenna is arranged close to the metal plate. This causes a problem that mutual coupling occurs and the current of the opposite phase in the current flowing in the antenna flows in the metal surface, and this changes radiation directivity characteristics of the antenna and substantially changes impedance characteristics, thereby breaking the matching state and deteriorating antenna gain deteriorates.
By contrast with this, a small radio of Patent Document 1 in which a whip antenna is provided in this upper part of its back surface, is provided with ribs for lifting the upper part side of the body of the small radio from a metal plate when the small radio body is laid and adjusting sensitivity characteristics of the whip antenna to prevent deterioration in antenna gain and realize good reception sensitivity. Further, a method is adopted for performing impedance matching on a metal plate by switching between antenna elements or between matching circuits by switching elements, or, as disclosed in Patent Document 2, a method is adopted for providing good antenna sensitivity even on a metal top panel by adding, in a position apart from an antenna power feeding point on the GND of the circuit board inside the housing, one or a plurality of elements which improve antenna characteristics, in which one end parts of the element parts are grounded to one point on the ground of the circuit board and in which the other end parts of the element parts are open ends.
However, with a method of providing ribs in the back surface of the above-explained small radio, the thickness of the body increases in proportion to the ribs themselves and, therefore, this makes it difficult to make the small radio thinner, and the ribs have significant impact on design and, therefore, this causes a problem of spoiling the outlook. Further, the method of switching between antenna elements or between matching circuits by switching elements, makes the circuit configuration and control complicated, and not only increases cost but also is not desirable for miniaturization of the small radio. Furthermore, with a configuration of adding one or a plurality of elements which improve antenna characteristics, in which one end parts of the element parts are grounded to one point on the ground of a circuit board and in which the other end parts of the element parts are open ends, new parts are added, and therefore this does not allow miniaturization (i.e. thinning, in particular) of the housing itself which is demanded in recent years.
Unlike the above-described terminals that need to change, for example, the size or design of the terminals and that deteriorate antenna gain when the body is laid on a metal plate, for example, Patent Document 3 discloses a mobile radio apparatus that employs a configuration of actively utilizing for antenna gain a metal top panel on which the mobile radio apparatus is placed.
Patent document 3 discloses a communication terminal employing a configuration arranging a conductor having an auxiliary ground plate, which is placed on a metal top panel, and a radiation element, which is orthogonal and adjacent to this auxiliary ground plate. With this communication terminal, in a state where the communication terminal is placed on a metal plate, mirror effects make a high frequency current of an opposite phase flow in the metal plate, so that the high frequency current is canceled and antenna radiation is weakened. However, the mirror effects do not make components vertical to the metal plate cancel each other, so that the radiation element forms a dipole antenna symmetrically with respect to the metal plate.    Patent Document 1: Japanese Patent Application Laid-Open No. HEI10-126304    Patent Document 2: Japanese Patent Application Laid-Open No. 2006-050496    Patent Document 3: Domestic Re-Publication of PCT international Publication for Patent Application No. 2004-042947